When multiple events (upvotes/downvotes/accept) happen on a single post, an expand icon shows up which shows all events in reputation tab. But since 2-3 days I observed that it ain't there. I thought it may be network wide but no, only Ask Ubuntu is affected.

Interestingly when I'm logged in, it doesn't shows up but when I'm logged out it's there.

Interesting. I observed exactly the same thing, with sites other than Ask Ubuntu whether or not I was logged in, and Ask Ubuntu when I was not logged in, on Post, and Ask Ubuntu when I am logged in being on Time. (That's how I'd describe it now, based on the insight in this answer, and checking which tab is selected.) Apparently the Post tab is the default but I'd clicked Time while using--and logged in to--AU. I have no memory of doing this, but there are a lot of things for me to click accidentally in the SE interface.
– Eliah KaganAug 20 at 11:33